Fix iFlytek scoring authentication dates

[HansonL622]

What changed

• Format the iFlytek Suntone WebSocket authentication date with a fixed two-digit day of month.
• Add regression coverage for both single-digit and two-digit calendar days.

Root cause

DateTimeFormatter.RFC_1123_DATE_TIME formats dates such as August 1 as Sat, 1 Aug 2026 .... The Suntone authentication endpoint requires EEE, dd MMM yyyy HH:mm:ss GMT, so requests made on days 1–9 were rejected during the WebSocket handshake with HTTP 403 before scoring began.

Impact

Pronunciation scoring can authenticate correctly on every day of the month. The change does not alter credentials, audio processing, score parsing, or frontend behavior.

Validation

• ./mvnw --batch-mode --no-transfer-progress -Dtest=IflytekScoringProviderDateTest,QwenRealtimeProviderTest test — 23 tests passed
• ./mvnw --batch-mode --no-transfer-progress clean verify — 188 tests passed
• Live Suntone probe with the configured local credentials — successful scoring response
• Existing backend CI runs ./mvnw --batch-mode --no-transfer-progress clean verify, so the new regression tests are included automatically

[fennoai]

No issues found in this review.

Verified: ./mvnw -Dtest=IflytekScoringProviderDateTest test and git diff --check 66db5d8a5bb9696ccc8678ce81e6f5b4c8618687...11224112245e6cfce7196452fbb138045fc40de4.

[yydounai1234]

LGTM